Re:iMessage? (Score 1)

iMessage can both send iMessages (show up as blue) and SMS (green) in the same app.
It automatically (mostly) determines if the recipient has iMessage and uses that preferentially (although you can say "send as text" on a per message basis).

As others have said, the "secret sauce" is convenience. A phone can proxy SMSs for an iPad or Mac using the same iCloud account so you can send and receive iMessages or SMSs from your laptop with zero configuration (apart from signing into iCloud which you do once on setup)

Also, that proxying stuff works for phone calls - you can answer a (real GSM, non-Facetime) phone call on your laptop or iPad (although I would agree if you thought this a fairly pointless feature)